The removal of grease films from metal surfaces by the application of grease solvents has been widely practiced during recent years. Numerous solvents have been employed, and numerous variations of the degreasing procedure have been suggested. In one method of operating, the metal article to be degreased is brought into contact with the solvent in the liquid phase. This may be accomplished by immersing the article in a large body of the solvent, or by spraying the solvent on the surface of the article. In another common method of degreasing, known generally as vapor-phase degreasing, a body of solvent is maintained at the boiling point and in communication with a chamber adapted to contain a large body of the solvent vapor. The article to be degreased is brought into contact with this body of vapor, and causes condensation of the solvent on the greasy metal surface. The condensed solvent removes grease and oil from the metal surface and drips off, usually returning to the boiling body of the solvent. With it goes dirt, adhering to the greasy surface. This dirt often includes metal chips.
Numerous solvents have been employed in degreasing operations. Among these, solvents of the chlorinated hydrocarbon type, including both saturated compounds such as carbon tetrachloride, and unsaturated compounds such as trichloroethylene and perchloroethylene, have been widely employed because of their high grease-solvent capacity and low flammability. Trichloroethylene is probably most widely used in degreasing. However, it is seldom used without the addition of some "stabilizer," adapted to prevent or retard its decomposition during storage and normal use. This so-called "normal" type of decomposition is promoted by light and oxygen. The action of light and oxygen is accelerated by heat. Several stabilizers are available and commonly used for the purpose of inhibiting this decomposition.
Pyrrole and N-alkyl pyrroles, especially those in which the alkyl group contains 1 to 4 carbon atoms, are the preferred stabilizers for this purpose. Examples of these are pyrrole, N-methyl pyrrole, N-ethyl pyrrole, 2-methyl pyrrole, 3-methyl pyrrole, 2,4-dimethyl pyrrole, 2,5-dimethyl pyrrole, N-propyl pyrrole and 2-chloropyrrole. The use of these stabilizers was first disclosed in U.S. Pat. No. 2,492,048. Another well-known inhibitor or stabilizer for trichloroethylene or perchloroethylene when used as a degreasing solvent is a group of organic esters. These are esters formed by the reaction of an aliphatic alcohol with an aliphatic carboxylic acid, each of which alcohol and acid can contain up to six carbon atoms. Specific examples include ethyl acetate, isopropyl acetate, butyl hexanoate, amyl acetate, n-butyl formate and the like. The above are disclosed as useful for stabilizers in U.S. Pat. No. 2,371,647.
A combination stabilizer is disclosed in U.S. Pat. No. 2,818,446 wherein esters such as the above are combined with an epoxide such as propylene oxide, butylene oxide or epichlorohydrin. Ethyl acetate and epichlorohydrin is a preferred combination.
Epoxides are also used in combination with amines as disclosed in U.S. Pat. No. 2,797,250. Amines both aliphatic, such as triethylamine, and aromatic, such as pyridine and the picolines, are useful in combination with epoxides such as butylene oxide or epichlorohydrin.
Another combination stabilizer, described in U.S. Pat. No. 2,906,783, consists of an epoxide, an ester, an alkene hydrocarbon and an azine. For example, trichloroethylene was stabilized by the addition of butylene oxide, isopropyl acetate, trimethylpentene and acetalazine. Other useful azines are derivatives of aliphatic aldehydes, e.g. propionaldehyde azine or butyraldehyde azine. Pyrazine and nitriles have been used to stabilize methylchloroform as disclosed in U.S. Pat. No. 3,798,170 and 3,564,063, respectively.
The present invention is a new stabilizer combination in which N-methyl pyrrole is employed with (1) an ester, e.g. ethyl acetate, or (2) a nitrile, e.g. acetonitrile, or (3) pyrazine in trichloroethylene or perchloroethylene when used in degreasing operations.